Financial Instruments - Additional Information (Detail) - USD ($)	Sep. 30, 2015	Dec. 31, 2014	Sep. 30, 2014
Fair Value Disclosures [Abstract]
Long-term debt, carrying values	$ 1,705,152,000	$ 1,585,395,000	$ 1,618,275,000
Long-term debt, fair values	$ 1,781,152,000	$ 1,680,584,000	$ 1,707,920,000